This invention relates to a hydrostatic pressure worm-rack device utilized in a machine tool, for example.
Prior art hydrostatic pressure worm-rack devices can be classified into two types. In one type, hydrostatic pressure oil is supplied from the side of the rack, and in the other type, the hydrostatic pressure oil is supplied from the side of the worm. Examples of the former type are disclosed in U.S. Pat. Nos. 3,789,956 and 4,148,227 and examples of the latter type are disclosed in U.S. Pat. Nos. 3,595,100 and 3,763,959. The patents illustrate a construction in which the hydrostatic pressure oil is distributed from the end surfaces of a worm and a construction in which thrust forces generated in hydrostatic pressure oil pockets provided for left and right ends of a worm are caused to balance with each other.
A hydrostatic pressure worm-rack device is used in a machine tool as a feed device. With recent increase in the feed speed of the machine tool, the number of revolutions of the worm is also increased so that lubricating oil in oil pockets provided for worm teeth flanks which do not engage with the teeth of a rack is caused to splash away by centrifugal force so that when the worm subsequently engages with the rack, air would be entrapped in the oil pockets which causes a pressure variation in the oil pockets, thus failing smooth high speed feeding.
Furthermore, according to the prior art construction, since no pilot oil supply ports and pilot oil discharge grooves are provided at both ends of the worm at which the worm begins to mesh with the rack, oil pressure would rise in the oil pockets so that air remaining in the oil pockets would not be discharged with the result that the pressure in the oil pockets pulsates which prevents smooth feeding.
According to the prior art constructions, at the time of supplying operating oil to the supply ports opening in the tooth flanks, since the operating oil is supplied through one end of the worm, pressure loss occurs before the operating oil reaches the supply ports at the far end. Accordingly, it is impossible to obtain uniform pressure in respective oil pockets, thus failing high speed feeding. Moreover, no means was provided for maintaining constant the pressure of the operating oil in respective oil supply ports of the tooth flanks.